Using the Textie [iTunes link] iPhone messaging app, you can send free unlimited messages to other Textie users, as well as to US numbers, and contact just about anyone provided you have their email address. Textie makes it possible for you to keep in touch with your friends in a variety of ways for free, using an app that is similar in appearance to the iPhone’s native SMS app.

Once you’ve downloaded the app, you have to register for a new account, entering your name as it will appear in messages you send, your email address and a password. After activating your account, you’ll be ready to start receiving and sending messages entirely for free.

Thanks to the the iPhone’s push notification feature, Textie is the ideal alternative to your native SMS app.

When you receive a new message, regardless of how it’s sent through Textie, you will receive a notification on your phone.

Texting actual phone numbers is currently limited to the US. The carriers that Textie supports are AT&T, Verizon, Sprint, Nextel, US Cellular, Metro PCS, Qwest and Virgin Mobile. They used to support T-Mobile although the service is now hit and miss.

Free messaging to US numbers is possible using Textie because the application sends its messages through email gateways, which might not make most carriers very happy. For US residents, in the account settings, add your US phone number to be sure to receive Textie messages sent to your phone number.

You can send messages to any email addresses, and the recipient’s responses will appear on your phone the same way an SMS would appear, making it extremely handy for non-iPhone users to instantly reach you using nothing more than email. Each message sent from Textie uses a unique email address to which users reply to – making it impossible for you receive spam through this iPhone messaging app.

For those of you with multiple email addresses, you can add as many addresses to the app as you like, and choose which one will be used when you send a message using Textie via email.

One of Textie’s useful features is the ability to block users you don’t want to receive messages from using the free app. All you have to do is reply to a message from them with the word ‘block’ and they can no longer contact you via Textie.

Unlike other similar apps available in the app store, you can also send pictures from your phone for free, by either taking a new photo or choosing an image from your camera roll. Textie works over WiFi, 3G and Edge.

What app do you use on your iPhone to keep in touch with your friends? Let us know in the comments.

Nancy is a writer and editor living in Washington DC. She was previously the Middle East editor at The Next Web and currently works at a DC-based think tank on communications and social media outreach.